This invention relates to an improved process for the preparation of p-aminophenol by the catalytic hydrogenation of nitrobenzene. p-Aminophenol is a well-known and very useful industrial chemical. It is used as an intermediate in the production of pharmaceuticals such as acetaminophen, in the production of dyestuffs such as sulfur dyes, and in making photographic chemicals.
There are many well-known methods for the production of p-aminophenol. The main processes involve the reduction of nitrophenol or the reduction of nitrobenzene in dilute sulfuric acid over platinum catalyst as shown by the following reactions: ##STR1##
Reaction I suffers from the disadvantage of higher raw material cost, while Reaction II is sensitive to changes in reaction conditions, e.g., temperature, hydrogen pressure, mode of addition and acid concentration, which significantly influence both the yield and quality of p-aminophenol isolated.
In the prior art, U.S. Pat. No. 2,198,249 (Du Pont) discloses the use of high temperatures, e.g., 135.degree.-155.degree. C., and concentrated sulfuric acid for the production of p-aminophenol. The hydrogen pressure is from about 400 to about 500 psi. The yields of the product, however, are relatively low and the final product of poor purity.
In U.S. Pat. No. 2,765,342 (Du Pont), nitrobenzene is reduced catalytically to p-aminophenol under a partial pressure of hydrogen of less than 760 mm Hg with the careful addition of nitrobenzene to the reaction mixture so that the unreacted nitrobenzene does not exceed its solubility in the reaction mixture.
U.S. Pat. No. 3,383,416 (Benner) shows the problem of recovery of the expensive platinum catalyst from the hydrogenation of nitrobenzene to obtain p-aminophenol. This is accomplished by terminating the reaction when about 85-95% nitrobenzene has reacted.
Other processes, such as disclosed in U.S. Pat. No. 3,535,382 (CPC International), show the use of a surfactant or a phase transfer catalyst to facilitate the solubilization of the nitrobenzene to avoid the necessity of adding the reactants in stages.
In all of the aforesaid methods, the p-aminophenol obtained as the end product is relatively impure and the operation is difficult, especially in large scale production.